1. Field of the Invention
This invention relates to a projection apparatus, and is suitable for a color liquid crystal projector for enlarging and projecting, for example, the original picture of a projected image displayed on a color liquid crystal panel onto the surface of a screen.
2. Related Background Art
There have heretofore been proposed various projection apparatuses (liquid crystal projectors) adapted to enlarge and project the original picture of the projected image of a liquid crystal light valve or the like onto the surface of a screen.
Apparatuses of various types are used as projection apparatuses, and in a projection apparatus using color liquid crystal as the original picture of a projected image, a color combining system such as a reflecting mirror or a dichroic mirror is disposed in the space (back focus) from the last lens surface of a projection lens to a liquid crystal display element to thereby effect the color combination of a colored imaged.
In a color liquid crystal projector using three liquid crystal panels for R, G and B lights as a color liquid crystal projector, use is made of a color combining system for combining color lights transmitted through the three liquid crystal panels into an optical path.
As the color combining system, for example, in Japanese Laid-Open Patent Application No. 1-131593, use is made of a so-called cross dichroic prism comprising four triangular prisms cemented together, and a dichroic surface being designed to cruciformly cross the cemented surface thereof, and this cross dichroic prism is provided in an optical path leading from a liquid crystal panel to a projection lens.
Generally, when a color combining system is comprised of a prism, the thickness of a dichroic surface is very small and its influenced on the optical performance (imaging performance) of a projection lens is very little, and this is suitable for projecting an image of high image quality.
On the other hand, FIG. 7 of the accompanying drawings is a schematic view of the essential portions of a color liquid crystal projector using three dichroic mirrors and a dichroic prism which is proposed in Japanese Laid-Open Patent Application No. 9-211750.
In FIG. 7, of the lights emitted from a light source 201 using a metal halide lamp, red light (light of about 600 nm to about 700 nm) is reflected by a red reflecting dichroic mirror 202 and the other lights are transmitted therethrough. The reflected red light has its travelling path changed by a reflecting mirror 203 and enters a liquid crystal light valve 206 for red. Of the lights transmitted through the red reflecting dichroic mirror 202, green light (light of about 500 nm to about 600 nm) is reflected by a blue transmitting dichroic mirror 204, and blue light (light of about 400 nm to about 500 nm) is transmitted therethrough. The green light reflected by the dichroic mirror 204 enters a liquid crystal light valve 205 for green, and the blue light transmitted through the dichroic mirror 204 enters a liquid crystal light valve 207 for blue. Each liquid crystal light valve has a polarizer 221 and an analyzer 222 mounted thereon, whereby light is modulated for each color to thereby form an image.
The blue light light-modulated by the liquid crystal light valve 207 for blue is reflected by a reflecting mirror 208, and is made incident on a blue transmitting dichroic prism 227 to be transmitted therethrough, and is enlarged and projected by a projection lens 211. The green light light-modulated by the liquid crystal light valve 205 for green is made incident on a red transmitting dichroic mirror 209 to be reflected therefrom, and is made incident on the blue transmitting dichroic prism 227 to be reflected therefrom, and is enlarged and projected by a projection lens 211. The red light light-modulated by the liquid crystal light valve 206 for red is made incident on the red transmitting dichroic mirror 209 to be transmitted therethrough, and is made incident on the blue transmitting dichroic prism 227 to be reflected therefrom, and is enlarged and projected by the projection lens 211. The combination of the red light and the green light is effected by the dichroic mirror 209, and the combination of the aforementioned combined light and the blue light is effected by the dichroic prism 227.
Liquid crystal projections in which a projection lens is comprised of a telecentric system are proposed, for example, in Japanese Laid-Open Patent Application No. 8-122699, Japanese Laid-Open Patent Application No. 10-10467, etc. Japanese Laid-Open Patent Application No. 8-122699 proposes an image projecting apparatus comprising a light source, a spatial light modulating element, an illuminating optical system for applying the light from the light source to the spatial light modulating element, and a projection optical system for projecting the image of the spatial light modulating element, and which is comprised of an illuminating optical system or a projection optical system. Japanese Laid-Open Patent Application No. 10-10467 proposes a projection display apparatus comprised of a light source device, an optical integrator for converting the light from the light source into a uniform light beam, a reflection type liquid crystal panel for modulating the light beam from the optical integrator and displaying an image, and a projection lens for enlarging and projecting the image displayed on the reflection type liquid crystal panel onto a screen.
A cross dichroic prism for color combination or color resolution suffers from the disadvantage that the deviation of liquid crystal pixels forming an image on a screen is caused by the inclination or level difference of two cemented surfaces when two prism surfaces are cemented together to form a dichroic surface performing a function (e.g. reflecting red light) into the two prism surfaces.
This disadvantage appears more remarkably as the number of the pixels of the liquid crystal used becomes greater and therefore, to solve this problem, the cementing step of very high accuracy becomes necessary, and it is feared that the costs of parts are increased.
On the other hand, in the liquid crystal projector shown in FIG. 7, a dichroic surface near the projection lens 211 is comprised of a prism and is made into a dichroic prism 227, and dichroic surface and reflecting surfaces near the liquid crystal panels 205, 206 and 207 are comprised of a dichroic mirror 209 and a mirror 208, respectively. When such a construction is adopted, the problem in the manufacture of the dichroic surfaces in the dichroic prism is avoided and the weight is not increased. However, in the optical path transmitted through the dichroic mirror 209, the dichroic mirror 209 is disposed obliquely relative to the optical path and therefore, there is the problem that astigmatism by the thickness of the dichroic mirror and the distortion of an asymmetrical image occur and in the optical path reflected by the dichroic mirror 209, the distortion of an asymmetrical image due to the distortion of the reflecting surface occurs.
It is written that in the color liquid crystal projector shown in FIG. 7, the deterioration of performance occurs to the reflecting optical path and transmitting optical path of the dichroic mirror 209, but the specific substance of the deterioration is not mentioned. The setting of the optical paths when a dichroic mirror is used need be considered with a phenomenon occurring in this dichroic mirror taken into account, and this will now be described with reference to FIGS. 8 and 9 of the accompanying drawings. The distortion of an image occurring in the transmitting optical path is such that when as shown in FIG. 8, the principal ray of light is incident on the dichroic mirror 209 at different angles, the amounts of shift (.DELTA.'1, .DELTA.'2 and .DELTA.'3) of the ray of light by the thickness of the mirror differ from one another and non-uniform distortion occurs to a projected image. On the other hand, in the reflecting optical path, as shown in FIG. 9, due to the distortion of the surface, the direction of reflection of the incident principal ray of light deviates from a predetermined direction and non-uniform distortions (.delta.1 and .delta.2) occur to the projected image. These distortions of the image occur asymmetrically though due to different causes and therefore, even if the liquid crystal panels are moved, the deviation of the pixels of the liquid crystal panels 206 and 205 cannot be corrected.
Now, there has been the problem that if in a construction wherein dichroic film is sandwiched between two members of glass, an attempt is made to increase the number of layers of the film and improve the optical performance of the dichroic film, and fluctuation of the half value cut wavelength of the dichroic film by the angle of incidence on the dichroic surface becomes great and color irregularity occurs in the picture plane of a projected image.
Against such a problem, the applicant of the basic application filed in Japan has proposed in Japanese Laid-Open Patent Application No. 7-294924 a construction of dichroic characteristic for suppressing color irregularity effectively in a liquid crystal projector using a dichroic mirror.
In the above-mentioned publication, description is not made of a liquid crystal projection having dichroic film sandwiched between two members of glass.